1. Field of the Invention
The present invention relates to a technique for correcting output light-amounts of a spatial light modulator with diffraction grating structure and the special light modulator is preferably used for recording an image.
2. Description of the Background Art
Developed has been a light modulator element of diffraction grating type which is capable of changing the depth of grating. The light modulator element is produced using a semiconductor device manufacturing technique and is alternately provided with fixed ribbons and moving ribbons on a substrate. The moving ribbons can be sagged relatively to the fixed ribbons to change the depth of grating. It is proposed that such a light modulator element is used for an image recording apparatus in techniques such as CTP (Computer to Plate) as an optical switching element, since the intensities of specular reflection light and diffracted light beams are controlled by changing the depth of grooves on the diffraction grating as above. Published Japanese translation of PCT. No. 2002-503351 discloses a light modulator element where fixed ribbons are formed on a substrate and moving ribbons are formed at positions away from the substrate.
The light modulator element of diffraction grating type has characteristics that an outputted light amount can be continuously changed. In a spatial light modulator having an array of such light modulator elements, an output light-amount diffracted from each light modulator element in an ON state and an OFF state can be equalized even if light from a light source has slight unevenness or physical properties are slightly different between the light modulator elements.
In the mean time, since a zeroth order light beam (specular reflection light) is outputted when an input voltage is 0 and a light amount of the zeroth order light beam is maximum in a case of a light modulator element of diffraction grating type where the height of the moving ribbons is the same as that of the fixed ribbons when the moving ribbons are not sagged, unevenness in the output light-amounts between the light modulator elements can be corrected with reference to the state of the light modulator element at an input voltage of 0. On the other hand, in a case of a light modulator element of diffraction grating type where the height of the moving ribbons is different from that of the fixed ribbons when the moving ribbons are not sagged, since the zeroth order light beam first becomes maximum after inputting a certain amount of voltage, a plurality of input voltages for acquiring a desired light amount may exist in the vicinity of a voltage where the light amount of the zeroth order light beam is maximum and therefore, it is not possible to easily determine an appropriate input voltage.